The invention relates to selenium rectifiers and more particularly to such structures having junctions employing crystalline selenium which are suitable for use as blocking contact for holes in photosensitive devices.
The electrical characteristics of vitreous selenium and hexagonal selenium (also technically known as "trigonal" selenium) vary considerably as discussed in an article entitled "Some Investigations on the Electrical Properties of Hexagonal Selenium" by L. M. Nijland, in Phillips Res. Rep. 9, (1954), pp. 259-294. For example, vitreous selenium at 20.degree. C. is highly resistive, i.e. its resistivity is between 10.sup.10 ohm-cm. and 10.sup.14 ohm-cm., whereas hexagonal selenium is quite conducting and may have a resistivity of from a few hundred ohm-cm. to 10.sup.9 ohm-cm. depending upon the concentration of halogens, or thallium impurities, respectively.
Nonphotosensitive selenium rectifiers are known which generally consist of a metallic base plate, a layer of hexagonal selenium, a barrier layer along the selenium layer forming a rectifying junction therewith, and a counter electrode. The barrier layer usually consists of cadmium selenide. Such rectifiers are described for example in:
U.S. Pat. No. 2,908,592, issued to H. Strosche on Oct. 13, 1959;
U.S. Pat. No. 3,473,095, issued to G. H. Griffiths on Oct. 14, 1969;
U.S. Pat. No. 3,622,712, issued to R. M. Moore et al. on Nov. 23, 1971 and in
"Electrical Behavior of the Contact Between Cadmium and Single Crystal Selenium Film" by C. H. C. Griffiths and H. Sang in The Physics of Selenium and Tellurium, W. C. Cooper Ed., Pergamon Press, Inc., London, England 1969, pp. 349-370.
In contrast to selenium rectifiers, a variety of photosensitive devices such as imaging pickup devices (e.g. camera tubes and electrophotographic plates) have been fabricated having photoconductive layers consisting of the highly resistive vitreous or amorphous form of selenium. Such devices, are for example, described in:
U.S. Pat. No. 3,350,595, issued to W. M. Kramer on Oct. 31, 1967;
U.S. Pat. No. 3,405,298 issued to J. Dressner on Oct. 8, 1968;
U.S. Pat. No. 3,861,913 issued to C. Chiou on Jan. 21, 1975; and
Great Britain Pat. No. 1,343,197 issued to Tanaka et al. and filed on June 14, 1971.
As a consequence of significantly differing application requirements, the prior art relating to selenium rectifiers has not been considered by persons of skill in the art of photosensitive devices as particularly relevant to photosensitive devices. For example, because of the extremely low dark current (e.g. 10nA. at 20 volts) and low forward current requirements of imaging devices, such devices have employed vitreous selenium because of its high resistivity and/or its ability to form a blocking contact with n-type semiconductor materials such as, cadmium selenide, or low work function oxidized metals such as oxidized aluminum. In contrast, nonphotosensitive selenium rectifiers have generally employed the low resistivity hexagonal polycrystalline selenium because of its less critical reverse leakage and higher forward current requirements. In general, hexagonal selenium layers have not been employed in imaging devices because of an inability to control dark or reverse leakage currents associated with its lower resistivity. Furthermore, certain techniques employed in manufacture of selenium rectifiers (e.g. surface roughening employed to obtain adhesion between a polycrystalline selenium layer and an electrode surface) are generally inapplicable to photosensitive devices.
Prior art electrophotographic plates are known having both vitreous and polycrystalline selenium layers in a single multilayered structure. One such structure is described in an article entitled "Advances in Xerography: 1958-1962" by C. J. Claus in Photographic Science and Engineering, Vol. 7, No. 1, Jan.-Feb. 1963. The structure therein described includes a tin oxide coating upon which two selenium overlayers are successively disposed. The first overlayer comprises an electron injection photoconductive layer of crystalline selenium and the second comprises a charge storage layer of amorphous selenium. In the operation of the structure as an electrophotographic plate, electron-hole pairs are photogenerated in the first overlayer and the electrons are injected into the highly resistive second layer where the electric field associated with stored surface charges, accelerates them to the surface. The structure described, however, is relatively complex and is not generally adaptable to some imaging devices, such as pickup tubes, requiring a blocking contact for holes.
In general, a thermally stable and easily manufactured rectifying contact is desired which is suitable for incorporation in imaging devices and other photosensitive devices as a blocking contact for holes.